1. Field of the Invention
This invention relates to a measuring circuit for the additive phase noise characteristic of a component in the vicinity of a carrier frequency.
2. Discussion of the Background
An additive phase noise (in contrast to a source phase noise) is the phase noise or jitter added by a given component of a circuit.
Presently, it is known to measure the additive phase noise produced by a frequency divider, which typically is a component for which the phase noise is a critical characteristic. The metrological engineering used so far for this purpose consists in producing, by a single generator, a predetermined frequency which feeds jointly and in quadrature two identical channels which each comprise a model of the component to be characterized. The signals obtained downstream from these components in each of the channels are applied to respective inputs of a mixer combining these two signals and delivering at the output a function signal of the instantaneous phase deviations between the two channels; the signal at the output of the mixer, which then is detected and amplified, thus gives an indication of the additive phase noise of the component to be characterized, the source phase noise of the generator, which is identical on the two channels, being eliminated by the mixture.
The limits of this method, which is based on the principle of the single phase detection, stem from the fact that at the output of the mixer there is generated not only a signal representative of the additive phase noise introduced by the component to be characterized but also of an additive noise superadded by the mixer (and its associated detection and amplification circuits) and by that of outside disturbances.
Now, concerning the mixer, it is very difficult to quantify its characteristic additive phase noise, which is not constant and varies in particular as a function of the signal levels applied to its inputs.
Further, concerning the outside disturbances, measuring circuits produced on the above-mentioned principle are extremely sensitive to radiation and vibrations, because the area in which it is sought to characterize the phase noise is located in a narrow frequency band around a given central frequency (designated as the "carrier frequency" here), typically in a range of 0.1 to 10 kHz around this carrier frequency. To compensate for those low frequency signals corresponding to high amplitude man-made noise signals induced by the environment, in particular radiations and vibrations (which induce the noise in the circuit by microphonic effect), it is necessary to provide extremely elaborate nonvibrating assemblies and shields and to perform the characterization of the component in a metrological environment with a low level of outside disturbances. This considerably increases the cost of the apparatus and limits its possibilities of use.
Thus, in practice, these components can not be accurately characterized in terms of phase noise, as the characteristic phase noise of the component is combined with the superadded phase noise of the mixer and the outside disturbances, and it then is very difficult to extract the signal that is desired to be measured from these mixed interferences.